The present invention relates to electric switches and, more particularly, to a safety press-button switch, which uses a linkage to control on/off switching of a bimetal plate, enabling the bimetal plate to automatically trip off upon an overload.
A variety of power switches are known for controlling on/off of power supply. Earlier power switches are seesaw switches, having ON/OFF function only. A seesaw switch does not trip off automatically upon an overload. FIGS. 1A and 1B show a seesaw switch according to the prior art. This structure of seesaw switch 1 comprises a cap-like switching lever 11 balanced on a support at its center and controlled to move a switching metal contact plate 12 between ON position and OFF position. The switching metal contact plate 12 automatically trips off upon an overload.
FIGS. 2A and 2B show a press-button type safety switch according to the prior art. This structure of safety switch comprises a housing 21, a press-button 22 supported on a spring 27, an actuating block 23 connected to the press-button 22, the actuating block 23 having a heart-shaped locating groove 24 at the right side, a bimetal plate 26 fastened to a right-sided power terminal 25, the bimetal plate 26 having a top pin 27 adapted to engage the locating groove 24. When pressing the press-button 22, the locating groove 24 of the actuating block 23 is forced into engagement with the top pin 261 of the bimetal plate 26 to close the circuit. In case of heat is produced and transmitted from the right-sided terminal 25 to the bimetal plate 26 due to an overload, the bimetal plate 26 is deformed to disengage the top pin 261 from the locating groove 24 of the actuating block 23, and the spring 27 immediately forces the press-button 22 upwards to OFF position. However, this structure of safety switch is still not satisfactory in function. When the metal contact holder 29 is lowered with the press-button 22 to force its contacts 291;292 into contact with the contact 281 of the middle terminal 28 and the contact 251 of the right-sided terminal 25, electric current is not connected to the bimetal plate 26, and the bimetal plate 26 is not directly sensitive to the electric current. When a short-circuit high temperature is produced, it is transmitted from the metal contact holder 29 to the right-sided terminal 25, and then transmitted from the right-sided terminal 25 to the bimetal plate 26. Because the transmission of heat from the metal contact holder to the bimetal plate 26 takes much time, the bimetal plate 26 does not trip off immediately upon an overload. Further, the four-contact (281;291;292;251) design is complicated. There are also known other similar safety press-button switches that automatically trip off upon an overload. However, these conventional safety press-button switches are commonly complicated, and expensive. In case one element fails, the actuating metal contact plate cannot trip off automatically upon an overload.
The present invention has been accomplished to provide a safety press-button switch, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a safety press-button switch, which uses a reversible linkage to control on/off status of a bimetal plate, enabling the bimetal plate to trip off automatically upon an overload. It is another object of the present invention to provide a linkage for a safety press-button switch, which fits any of a variety of on/off switching metal contact plates. It is still another object of the present invention to provide a safety press-button switch is easy to install and, durable in use. According to one aspect of the present invention, the safety press-button switch comprises a linkage formed of a lever and a switching rod pivoted together, and a press-button cap having a push rod and a hook arm respectively aimed at the switching rod and a positioning rod of the lever and adapted to force the lever to shift an actuating metal contact plate from OFF position to ON position and to hold the lever in position after the actuating metal contact plate switched on, the positioning rod of the lever being forced away from the hook arm by the deformation force of the bimetal plate for enabling the actuating metal contact plate to trip off automatically upon an overload. According to another aspect of the present invention, the actuating metal contact plate is a bimetal plate.